Fluid control valves are often utilized for the on/off control of the flow of fluid or for precisely controlling or throttling the amount of fluid, such as in a gas or oil pipeline system in response to valve controllers and actuators. The precise control of fluid is desired in such systems in order to control the delivery of a precise amount of fluid, or in order to conserve the application and use of fluid additives which may be extremely dangerous or highly expensive. The control valve also must provide a reliable sealing shut-off of the fluid flow when the valve is placed in the shut-off position to prevent the undesired loss of fluid through leakage.
Presently available fluid flow control valves are rotary action control valves with a rotating ball control member mounted in a valve body passageway and are operated by a valve actuator such as a pneumatic piston or diaphragm actuator responding to the output of a valve positioner for throttling control of the valve and thereby to provide desirable accurate throttling control of the fluid flow stream. The rotating ball control member is mounted in a valve body passageway and suitable fluid sealing members are provided between the ball and the valve body.
In such present fluid control valves there is a certain amount of inherent lost motion between controlling movement of the valve operating member in responding to the valve positioner and the actual rotational movement of the ball control member. This lost movement leads to a diminished accuracy of throttling control. In some instances a tight connection is provided between the valve operating member and the ball control member in an attempt to reduce the lost motion between the ball and the operating member. However, in such valves the hydrostatic pressure exerted on the valve shaft tends to move the ball control member away from a reliable sealing position in the valve body passageway which inevitably leads to increased wearing of the fluid seals and reduced seal life.
Another problem in presently available fluid control valves is the inability to readily replace the seal members during normal valve maintenance operations. Typically, present throttling control valves must be substantially disassembled in order to replace the seal members which leads to increased maintenance expenses and increased downtime in the pipeline system.
Accordingly, it is desired to provide a fluid control valve having superior throttling control, which is unaffected by hydrostatic pressure on the valve shaft to increase fluid seal reliability, and wherein the fluid seals can be readily replaced with a minimum of valve component removal.